This invention relates to a vapor tank using vapor as a heat transfer medium to heat the product to be processed.
Vapor phase systems have been developed which use vapor as a heat transfer medium and utilize its thermal energy to heat the product to be processed. For example, in one of those systems, a vapor tank generally referred to as a vapor reflow tank is used wherein an electronic component is mounted on a soldering pattern of a printed circuit board and the printed circuit board is passed through a vapor phase to heat and melt the solder to thereby achieve vapor phase soldering of the electronic component onto the printed circuit board. In the other of those systems, a vapor tank is also used for performing heating to cause a resinous material to set, and a vapor tank for effecting fluxless fusing of solder plating on a printed circuit board.
Several well-known types of vapor tank will be outlined. One of them has a vapor generating section located separately from the vapor tank for generating vapor which is introduced into the vapor tank through an upper portion thereof. The vapor tank is formed with an inlet and an outlet at its side wall. A conveyor or other conveying means is provided for conveying the product to be processed into and out of the vapor tank where the product is heated by the vapor enveloping it before being released from the vapor tank. This type of vapor tank, which suits applications using vapor of high specific gravity with respect to air, has been in use.
When the product is heated by vapor, it is necessary that the product be enveloped sufficiently by the vapor and it is essential that the time required for heating be minimized to avoid deterioration of the product while enabling thermal energy to be provided in an amount large enough to accomplish the object. To this end, it is desirable that the vapor for enveloping the product be saturated vapor.
The vapor tank of the type outlined hereinabove suffers the disadvantage that the vapor phase formed around the product is prevented from becoming saturated vapor by the fact that the vapor is diffused or air is drawn into the vapor due to a local turbulence of the vapor. It has been ascertained with regard to water vapor that the heat transfer speed of unsaturated vapor with respect to the product to be processed is lower by 30-50% than saturated vapor when the proportion of air entrained by the vapor is 2%. Owing to the fact that the vapor phase enveloping the product is not saturated vapor, this type of vapor tank has had many problems. More specifically, the heat transfer speed is low and the temperature of the product is prevented from rising to a predetermined level. If heating is continued until the temperature of the product reaches the predtermined level, then the time required for attaining the end is too long to avoid deterioration of the product which is not desirable. Particularly, the air tends to remain on the underside of the product, resulting in a rise in temperature becoming unsatisfactory.
Meanwhile, another type of vapor tank has a vapor generating section located inside the tank. This type of vapor tank is disclosed in the article entitled "In-line Vapor Phase Soldering" appearing in "ELECTRONIC PACKAGING and PRODUCTION", November 1982. In this tank, a liquid stored in a lower portion of the tank is heated by heaters to generate vapor which envelops the product to be processed, to heat the same. This type of tank also has to have openings for conveying the product into and out of the tank, and the vapor tends to flow out of the tank through these openings. Thus, to form and maintain a saturated vapor phase for enveloping the product requires an overwhelmingly larger amount of heat for generating vapor than for heating the product by the generated heat, so that this system is uneconomical.
The foregoing description refers to problems with regard to the vapor tanks for supplying vapor to the product to be processed. Another problem that has to be solved with regard to the vapor is how to recover the vapor that has been used. The vapor used in the vapor tanks described hereinabove has a molecular weight which is about 820 gram/mol at applied temperature and pressure, so that its specific gravity with respect to air is about twenty-fold. Such heat transfer medium is very expensive and it has hitherto been usual practice to use a recovery device for recovering the used vapor for reuse.
A recovery device for recovering used vapor popularly used comprises condensers located beneath passages providing an inlet and an outlet to the vapor tank for liquefying the vapor that flows out of the tank. The condenser is a device having built-in cooling tubes for cooling the vapor into a condensate, and communication tubes for returning the condensate to the lower portion of the vapor tank.
The vapor introduced into the condenser through an upper portion thereof is first brought into contact with the cooling tubes in the upper portion of the condenser and then falls by its own weight, so that it is brought into contact with one cooling tube after another until it reaches the bottom of the condenser. While the vapor falls in the condenser as described hereinabove, it is cooled and turned to a condensate which collects on the bottom of the condenser. At this time, the temperature distribution of the vapor or condensate is such that the temperature is high in the upper portion of the condenser and low in the lower portion thereof. That is, the vapor is high in concentration in the upper portion of the condenser and low in the lower portion thereof. Thus, the vapor near the upper portion of the condenser is high in temperature and concentration, so that the vapor recovering efficiency is low. Also, the temperature of the condensate progressively falls as it drops from the cooling tubes in the upper portion of the condenser to those in the lower portion thereof, and the condensate initially standing at 215.degree. C. is sometimes cooled such that its temperature is close to that of cooling water flowing through the cooling tubes when it reaches the bottom of the condenser. The condensate thus produced is returned through the communication tubes to the lower portion of the vapor tank to be revaporized. To achieve revaporization of the condensate would require not only sensible heat produced by the heaters for heating the liquid to its boiling point but also latent heat for vaporizing the liquid at its boiling point. In other words, the condensate overcooled by the condenser would require sensible heat for heating the liquid to its boiling point by the heaters. Thus, the amount of heat generated by the heaters would be wasted.
Another problem raised in this type of vapor tank is that when the product that has been processed in the vapor tank is conveyed from the tank to outside through the outlet passage, the vapor would be formed into frost on the surface of the product and the expensive liquid would be released to outside without being recovered if the temperature of the product fell below the dew point of the vapor in the outlet passage.